Strange behaviour: struct with cell values

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Simon Baeuerle 2022-6-27

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https://ww2.mathworks.cn/matlabcentral/answers/1748870-strange-behaviour-struct-with-cell-values

评论： Simon Baeuerle 2022-6-27

Hi there,

I'm writing a bigger piece of code and I am using structs as input variables. Now I changed (or tried to change) one input parameter in this struct from an array to cell. A little bit of code first:

order = 1;                  %Dumm
param = {1,2,zeros(2,2)};
opt1.order = order;
opt1.param = param;
opt2 = struct('order',order,'param',param);

As we can see from that bit of sample code,:opt1 is a 1x1 struct (which is what I want) and opt2 is a 1x3 struct (which I do ABSOLUTELY not want).

Why do the two lines of code behave differently?

Is there a way, that opt2 does not get expanded to a 1x3 struct?

Thanks so much!

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Answer 1

Stephen23 2022-6-27

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https://ww2.mathworks.cn/matlabcentral/answers/1748870-strange-behaviour-struct-with-cell-values#answer_994435

编辑：Stephen23 2022-6-27

在 MATLAB Online 中打开

"Strange behaviour: struct with cell values"

The STRUCT() documentation explains what happens with cell array inputs: "... if value is a scalar cell array, then s is a scalar structure... If value is a nonscalar cell array, then s is a structure array with the same dimensions as value."

So it is very simple: if you want the output to be a scalar structure, then the input must be a scalar cell array.

You can achieve this very easily by nesting your (non-scalar) cell array in a scalar cell array:

order = 1;                  %Dumm
param = {1,2,zeros(2,2)};
opt2 = struct('order',order,'param',{param})
opt2 = struct with fields:
    order: 1
    param: {[1]  [2]  [2×2 double]}

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Steven Lord 2022-6-27

The command opt1.param = param doesn't call the struct function. It performs indexed assignment. Indexed assignment into a struct array doesn't have the same behavior as calling the struct function.

Why does struct behave this way? That predates the start of my time at MathWorks, and I haven't done the archaeology to try to determine the reason for that behavior.

Simon Baeuerle 2022-6-27